Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a petrophysicist regarding the particular earth formation being drilled.
In conventional oil well wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with traditional techniques, various parameters of the earths formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
If knowledge of the formation properties are needed while drilling, sensors can also be deployed near the end of a drilling string. Measurements of formation properties can be measured and stored in memory for later retrieval and correlation with depth. Measurements can also be transmitted to the surface by pulses of mud pressure or other means. This process is referred to as “logging while drilling” (LWD).
For a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well-known physical characteristics. In general, the electrical resistivity (i.e., opposition to the flow of electrical current) of formations containing hydrocarbons is high, and, the electrical resistivity of formations containing water is relatively low. Thus, if the porosity of an earth formation is known from other sensors, its electrical resistivity can assist the petrophysicist in determining the volume fraction of hydrocarbons in the formation.
Electrical resistivity is primarily measured using two methods. The first method includes sensors that utilize electrodes to force current to flow through the formation. By measuring the amount of voltage differential between the electrodes (with the amount of forced current known), the resistivity may be calculated. The second method utilizes coil-type sensors, also referred to as “induction devices” or “coils”, to induce an alternating electromagnetic field in the formation. A rudimentary induction device arrangement comprises a two coil arrangement including a single transmitting coil and a single receiving coil. The electromagnetic field induced in the formation by the transmitting coil further induces alternating electric currents (or eddy currents) in the formation. These eddy currents in turn generate a secondary electromagnetic field in the formation, thereby inducing an alternating voltage at a receiving coil. The voltage measured by the receiving coil varies as the resistivity of the formation changes, and the volume fraction of hydrocarbons in the formation may be determined by measuring the voltage signal at the receiving coil.
Ideally, the signal received by the receiving coil only represents formation resistivity or conductivity. The transmitting coil also directly couples the transmitted signal into the receiving coil, however. In this manner, the signal received by the receiving coil comprises the signal from the surrounding formation as well as the signal that is directly coupled from the transmitting coil. The signal that is directly coupled between the transmitting coil and the receiving coil is a function of the mutual inductance between the two coils and contains no information about the formation. Also, the magnitude of the direct coupled signal can be several orders of magnitude larger than the signal from the formation, which may make it difficult to distinguish the desired formation signal from the undesired signal that is coupled directly from the transmitter. Thus, in order to measure the formation signal accurately, the direct coupling signal should be removed. The term “bucking” refers to removing or minimizing the portion of the received signal that is directly coupled into the receiving coil from the transmitting coil.
Traditionally, bucking is implemented by adding additional coils or “bucking coils” to the transmitting coil and/or the receiving coil. And, if the bucking coil is added to the transmitting coil, the same current flows in both the transmitting coil and the bucking coil.
An array-type induction tool consists of several sub-arrays. A sub-array may be defined as a set of coil configurations that is used to acquire elementary measurements of e.g., formation measurements in an array-type tool. The transmitter or transmitters in an array-type tool can be commonly shared across all sub-arrays. In some cases, a common receiver or common receivers can also be shared across all sub-arrays. Traditional methods and apparatus, however, do not have a bucking coil shared by the sub-arrays. Therefore, every sub-array has its own bucking coil or bucking coils.
FIG. 1A depicts a high resolution sensor array 1 for use in a resistivity tool. Sensor array 1 includes multiple sub-arrays, where each sub-array comprises the transmitter coil TM, at least one receiving coil RN, and at least one bucking coil RNB for each receiving coil; each receiving coil RN is spaced a predetermined distance away from the transmitter. The various sub-arrays use the same transmitter coil TM and each sub-array has its own bucking coil. Therefore, the number of bucking coils for sensor array 1 equals the number of sub-arrays. In this manner, a five coil “symmetrical” sub-array may be formed using the transmitting coil TM, two symmetrically placed receiving coils R1, and two symmetrically placed bucking coils R1B. Alternatively, a three coil “non-symmetrical” sub-array 2 also may be formed using the transmitter coil TM along with a single receiving coil R1 and the bucking coil R1B associated with the receiving coil R1. Note that in the various arrangements, each sub-array requires at least one bucking coil corresponding to each receiving coil. Since conventional tools normally contain several sub-arrays, multiple bucking coils are often required, yet implementing numerous bucking coils on the downhole tool can be problematic for several reasons.
For some downhole tools, finding a suitable space on the downhole tool for the bucking coils of all sub-arrays can be difficult. In addition, as more sub-arrays are added to the downhole tool, finding a suitable space on the downhole tool for each bucking coil of each sub-array can be difficult. For example, FIG. 1B depicts two five-coil sub-arrays, sub-arrays 3 and 4, which may be implemented as a single high resolution tool. As illustrated, the purpose of the configuration is to measure the formation conductivity at different depths of investigation (i.e., d1 and d2). As illustrated, the receiving coil RM1 for sub-array 3 is capable of receiving signals at an investigation depth of d1, and the receiving coil RM2 for sub-array 4 is capable of receiving signals at an investigation depth of d2. Since the physical separation between transmitting coil T2 and the receiving coil RM2 is greater in sub-array 4 than the separation distance between transmitting coil T1, and receiving coil RM1, in sub-array 3, sub-array 4 is capable of receiving signals at an investigation depth of d2, which is greater than the investigation depth d1 of sub-array 3. Sub-arrays 3 and 4, illustrated in FIG. 1B, correspond to different depths of investigation, and as such, the number of windings in the bucking coils (i.e., RB1, and RB2) may differ between sub-array 3 and sub-array 4. Hence, separate bucking coils may be required for high resolution arrays, which may add to the number of coils required on the downhole tool.
In addition, systems that utilize triple component transmit and receive coils (see e.g., U.S. Pat. No. 5,757,191 to Gianzero), may require extra bucking coils associated with each component of the receiving coil and further exacerbate this problem. For example, FIG. 1C illustrates a high resolution array 5 including a receiver coil 6 and a bucking coil 7, where the receiving coil 6 further comprises three component coils RX, RY, and RZ. Accordingly, the bucking coil 7 further comprises RXB, RYB, and RZB. In this manner, each additional three component receiver coil 8 will require a three component bucking coil 9 and increase the number of coils that must occupy the limited space on the downhole tool.
To achieve a sub-array with the direct coupling minimized, traditionally, there are only two variables, or two freedoms, of a bucking coil that can be adjusted, the position of the bucking coil or the number windings. This limited number of freedoms is the root cause of all the difficulties described above.